I decided to experiment with 4-2 a little more today. Still, it seems very unlikely that the final frame in that level will be saved.

Here is a TAS that I put together of the level using the most optimal routes and movement: http://dehacked.2y.net/microstorage.php/info/1138750230/SMB1Perfect4-2TAS.fm2

Doing the wall clip requires subspeed manipulation and specific movement in order to push Mario's screen position more. If the movement is not optimal, frames will be lost performing the clip and getting to x-position 132 ($3ad).

The one frame improvement to the warp zone compared to HappyLee's 2011 warps TAS (and every other speed-oriented SMB1 warps TAS from late 2004 onward) is barely possible and was subpixels away from not being possible.

I hope that the next frame is found, but I have my doubts about it being possible.

It is worth noting that the framerule save in 4-2 is possible with fire flower by using a faster clip, but unless it is possible to somehow despawn one of the piranha plants, it probably holds no use for any%.